Control of undesirable and detrimental deformation of foundation material is important for modern super-structures. The problem is still more important be hydraulic structures founded on gypsiferous soils. For a safe
design of foundations supported by gypsiferous soils, it is vital to determine the physical and mechanical characteristics of the soil during its leaching
process. For evaluation of consolidation properties of gypsiferous soils
after the process of leaching, three samples were taken from three regions in Iran, known for their gypsiferous soils (Vafraghan plain in Saveh; Nekoo ¬Abad and Abshar plain in Isfahan and Ghir - Karzin plain in Fars province).
The Samples. gypsum content were (16 - 22), (21 - 32) and (6 - 10) respectively. To investigate the process of leaching and fixation of gypsum in
the soils, a physical laboratory model was devised and compacted samples at optimum moisture content, were leached by distilled water under 75, 135 and
215cm. hydraulic head for a period of 90 days. To evaluate the possibility of fixation of gypsum by chemical compounds, a treatment was made using 1:100
solution of sodium oxalate as mixing water (instead of distilled water) and tested under 215cm. hydraulic head for a period of 90 days. In order to
investigate the effect of magnesium chloride on the leaching rate of gypsum, another treatment was leached by a 5% normal solution of magnesium choride
under 215cm. head, for a period of 90 days. The overall results of the experiments indicated,that gypsum leaching of soil samples from Isfahan,
Saveh and Shiraz with SM, SC ,and CL classification, were (13.0 - 43.1), (4.3 ¬30.1) and (2.4 - 9.4) percent, respectively. The leaching process increased the rate of consolidation (Cc) of the samples by (1.6 - 1.9), (1.1 - 1.2) and (0.7 ¬0.9) times, respectively. The coefficients of consolidation were also increased
to (1.9 - 87.8), (0.6 - 7.5) and (0.9 - 2.3) times those of the initial conditions, respectively. Application of sodium oxalate to mixing water had no
considerable effect on gypsum fixation. and magnesium chloride caused higher rate of gypsum dissolution, but finally reduced consolidation rate because of
salt deposition.
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